Method for operating a high sensitivity active pixel

ABSTRACT

A method of operating a high sensitivity active pixel for use in metal oxide semiconductor (MOS) image sensor circuits. Light is allowed to be incident upon a photodetector circuit to thereby generate an input signal that represents the light. The input signal is applied to a gate of a first field effect transistor (FET). A control signal is applied to a drain of the FET and thereby generates an output current at a source of the FET. Charge is accumulated in a capacitor that is coupled to the source of the FET, where a voltage across the capacitor represents the detected incident light.

This application is a Divisional of Ser. No. 08/987,176, filed on Dec.8, 1997, now U.S. Pat. No. 6,046,444.

BACKGROUND

This invention is related to electronic image sensor circuits builtusing metal oxide semiconductor (MOS) fabrication processes.

Image sensor circuits are used in a variety of different types ofdigital image capture systems, including products such as scanners,copiers, and digital cameras. The image sensor is typically composed ofan array of light-sensitive pixels that are electrically responsive toincident light reflected from an object or scene whose image is to becaptured.

FIG. 1 illustrates a prior art pixel 100 with electronic shutter thatmay be built using MOS fabrication processes. The pixel 100 includes aphotodiode PD₁₀ coupled to a RESET field effect transistor (FET) M₁₀with an electronic shutter mechanism provided by a SAMPLE transistor M₁₁and a storage capacitor C₁. In operation, the pixel 100 is reset byapplying a RESET signal which causes the RESET transistor to provide alow impedance path and thus reverse bias PD₁₀. Next, a SAMPLE signal isapplied to create a low impedance path between nodes A and B, therebycharging C₁ to a reset level that is typically close to the rail orsupply voltage V_(DD).

When the object or scene comes into view of the sensor circuit and theincident light is allowed to shine on PD₁₀, node A is isolated fromV_(DD) by deasserting the RESET signal, and the voltage at nodes A and Bbegins to decay. The rate of decay is determined by the photocurrentI_(PHOTO) in PD₁₀ (caused by light-generated electron-hole pairs), byany leakage current through PD₁₀, by the capacitance of C₁, by theparasitic capacitance of C_(D1), and by any parasitic leakage paths tothe nodes A and B (not shown).

After a predetermined interval, known as the exposure or integrationtime, has elapsed from the moment node A was isolated, node B is alsoisolated by deasserting SAMPLE, thereby capturing a light-generated“exposed” value at node B. The capacitance of C₁ is selected so that theexposed value may be held at node B until a related signal is read atthe OUTPUT node.

To read the OUTPUT node, an ADDRESS signal is applied to the transistorM₁₃ which acts as a switch to cause a signal related to the exposedvalue to appear at the OUTPUT node. The difference between the exposedvalue and a “reset” value (representing the reset voltage at node B)gives a measure of the amount of light intensity that was incident onthe pixel 100 during the exposure time. A similar but more complexread-out circuit having intra-pixel charge transfer is discussed in U.S.Pat. No. 5,471,515 to Fossum et al.

The performance of an image capture system depends in large part on thesensitivity of each individual pixel in the sensor array and itsimmunity from noise. Noise here is defined as small fluctuations in asignal that can be caused by a variety of known sources. Such sourcesinclude, for instance, undesirable leakage currents and manufacturingvariations between pixels. An image sensor with increased noise immunityyields sharper, more accurate images in the presence of noise.

Improving the sensitivity of each pixel permits a reduction in exposuretime which in turn allows the capture of images at a greater rate. Thisallows the image capture system to capture motion in the scene. Inaddition to allowing greater frame rate, higher pixel sensitivity alsohelps detect weaker incident light to capture acceptable quality imagesunder low light conditions.

Pixel sensitivity is defined here as being related to the ratio of achange in the pixel output voltage (e.g., at the OUTPUT node of pixel100) to the photogenerated charge in the pixel. For the prior art pixel100 in FIG. 1, the sensitivity also depends in large part on the totalcapacitance of node B. The capacitance at node B includes the sumcapacitance of C_(D1) and C₁ where C_(D1) is the photodiode diffusionparasitic capacitance. Reducing the sum capacitance increasessensitivity, because the change in the voltage at node B is increasedwhen the capacitance is smaller for the same photogenerated charge.Reducing the capacitance, however, lowers noise immunity in that thevoltage at node B is more susceptible to leakage currents.

Another way to increase pixel sensitivity is to increase the efficiencyof the photodiode by changing the photodiode responsitivitycharacteristics. Doing so, however, can require deviating from astandard MOS integrated circuit fabrication process, thereby furtherincreasing the cost of manufacturing the image sensor circuit.

It is therefore desirable to have a pixel design with improvedsensitivity and noise performance using electrical circuitry availablewith standard MOS fabrication processes.

SUMMARY

This invention in one embodiment is directed at a pixel havingphotodetecting, amplifying, and storage circuitry. The photodetectingcircuitry provides a first signal in response to incident light. Theamplifying circuitry receives a control signal and provides an outputcurrent in response to the first signal and the control signal, theoutput current being received by the storage circuitry.

In another embodiment of the invention, the pixel is part of an imagesensor of an imaging system. The system includes an optical system forbeing exposed to incident light, an image sensor coupled to the opticalsystem to receive the incident light, where each pixel in the sensor hasphotodetecting circuitry providing an original signal representative ofthe incident light, amplifying circuitry having a signal input, acontrol input, and an output, the amplifying circuitry providing anoutput current in response to receiving the original signal at thesignal input and a first control signal at the control input, andstorage circuitry coupled to the output for receiving the output currentand in response providing an exposed voltage representative of theincident light. The system also includes analog-to-digital conversioncircuitry coupled to the sensor for converting analog signals related tothe exposed voltage in each pixel into digital signals representing rawimage data, digital signal and image processing generating capturedimage data in response to receiving the digital signals, and an outputinterface for transferring the captured image data to an imageprocessing system separate from the imaging system.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features as well as advantages of the differentembodiments of the invention will be apparent by referring to thedrawings, detailed description and claims below, where:

FIG. 1 illustrates a prior art pixel.

FIG. 2 is a schematic of pixel according to a first embodiment of theinvention.

FIG. 3 is a timing diagram showing control signals used in variousembodiments of the invention as well as intermediate signals within apixel.

FIG. 4 is a circuit schematic of a pixel according to a secondembodiment of the invention.

FIG. 5 is a logical block diagram of a digital image capture systemaccording to another embodiment of the invention.

DETAILED DESCRIPTION

As summarized briefly above, an embodiment of the invention is directedat a high sensitivity active pixel in which the storage capacitor isseparated from the photodetecting circuitry by an isolation stageincluding an amplifying device. The amplifying device can be a MOS FETconfigured as a source follower. The pixel can also include a read-outstage that provides an output signal in response to a voltage across thecapacitor. Rather than acting as a switch as in the prior art pixel 100,the isolation stage acts as a high input impedance amplifier thatpermits the storage capacitance to be increased independently of thephotodiode. This permits more noise immunity from leakage currents,without the conventional drop in sensitivity encountered when increasingthe capacitance of the prior art pixel. Moreover, the pixel embodimentsof the invention exhibit an increase in sensitivity by a factor of twoor greater over the prior art pixel 100, for the same photodetector andstorage capacitance.

Operation of the various embodiments of the invention will be explainedusing a MOS implementation of the circuits. The following short cuts areused in this disclosure to describe various operating regions of theFET. An FET is said to be “turned off” when V_(GS) (gate-sourcevoltage)≦V_(T) (threshold voltage) for the device and the device isoperating in the cut-off region where its channel acts as an opencircuit. When a FET is “turned on”, V_(GS)>V_(T), V_(DS) (drain-sourcevoltage) is normally small and the device is operating in thenon-saturation region.

FIG. 2 illustrates a first embodiment of the invention as a pixel 200.The pixel 200 includes conventional photodetecting circuitry 204,including for this embodiment a photodiode PD₂₀ and RESET transistorM₂₀, which are also coupled to node A′ and amplifier B₂₄. The photodiodePD₂₀ includes a parasitic capacitance, represented as C_(D2). In thisparticular embodiment, the amplifier B₂₄ includes an n-channel FET M₂₁which is configured as a source follower when SAMPLE is provided as avoltage supply at its drain (or control input). The FET M₂₁ receives alight-generated signal at its gate (or signal input) at node A′, wherethe light-generated signal is the voltage across the photodiode PD₂₀ forthis embodiment. The source (or output node) of M₂₁ is coupled tostorage circuitry B₂₈ which in turn feeds an output stage B₂₆. Thestorage circuitry B₂₈ includes for this particular embodiment a singlestorage capacitor C₂ referenced to ground. The capacitance of C₂ can becomparable to C_(D2) or even larger, depending on the desiredsensitivity and noise immunity. The storage circuitry B₂₈ receives anoutput current from the amplifier B₂₄. The output current charges thestorage capacitor C₂ to a voltage that is representative of thelight-generated signal (voltage) at node A′.

In operation, the pixel 200 of FIG. 2 operates according to theexemplary timing diagram in FIG. 3. The circuit operation will bedescribed with reference to the time points in the time line of FIG. 3and the control and test signals of the pixel 200.

At time 0, the RESET transistor is turned on thereby charging thecapacitance C_(D2) at node A′ up to a voltage defined by {overscore(SAMPLE)} (the inverse of SAMPLE). Although node A′ will rise to a pointthat is sufficient to turn on M₂₁, as long as the SAMPLE voltage is keptclose to ground the storage capacitor C₂ will remain discharged.

At time 1, the RESET signal is toggled thereby turning off the RESETtransistor, starting what is conventionally known as the exposure orintegration time. Beginning at time 1 and ending at time 4, the voltageat node A′ decays as C_(D2) is discharged in response to a photocurrentin the photodiode PD₂₀. The photocurrent (and hence the decay rate atnode A′) is related to the light intensity incident upon the pixel 200.

At time 2, SAMPLE (and {overscore (SAMPLE)}) are toggled such that M₂₁is configured as a source follower thereby charging C₂ to a capturedvoltage V_(A′)−V_(T) at node B′, where V_(T) is a threshold voltage ofFET M₂₁. Thereafter, between time 2 and time 3, the voltage at A′continues to decay such that V_(GS)<V_(T), thereby tending to turn offM₂₁. The source follower M₂₁ operated in this manner thus acts like apeak detector.

At time 3, the ADDRESS voltage is toggled thereby turning on M₂₃. Avoltage or current at the OUTPUT node, related to the exposed voltage atnode B′, may then be read.

The total light energy incident upon the pixel during the integrationtime (time 1 to time 2) may be obtained by computing the area under thevoltage at node A′ between time 1 and time 2. This can be approximatedby using the exposed and reset values at the output node as theendpoints of the area to be computed. To obtain more accurate valuesacross the entire sensor array, the technique of correlated doublesampling (CDS) provides for reading a reset voltage for a “dark image”,and correlating the same with the exposed voltage for the exposed image.Thus, to obtain the reset value for the dark image, RESET is toggled attime 4 to turn on the RESET transistor M₂₀ again. Then, at times 5 andthen 6, {overscore (SAMPLE)} is toggled twice so as to generate thereset voltage at the OUTPUT node.

To help linearlize the relationship between the voltage at the OUT PUTnode and the voltage at node A′, the dimensions of transistors M₂₁ andM₂₂ (where M₂₂ is a p-channel FET in the embodiment of FIG. 2) as wellas the characteristics of the load (not shown) which is connected to theOUTPUT node can be adjusted by one skilled in the art in an attempt tocancel the gain distortion caused by the n-channel source follower (M₂₁)with the distortion introduced by the p-channel M₂₂. Also, thecombination of n-channel source follower M₂₁ and p-channel M₂₂ may beable to compensate for the V_(T) drop from the gate to the source ofeach transistor, so that in effect a net voltage gain may be obtained atthe OUTPUT node with respect to node A′.

The configuration of pixel 200 has been simulated and fabricated using astandard MOS process. It indeed exhibits higher sensitivity, andtherefore allows lower integration times than the prior art pixel 100for equal capacitances C₂=C₁. This in turn allows greater image framerate without the conventional loss in dynamic range at the OUTPUT nodesuffered by decreasing the integration time in the conventional priorart pixel 100.

Another advantage of the pixel 200 is an increase in totalsignal-to-noise ratio (SNR) over that of the prior art pixel 100 forequal capacitances C₁=C₂. The SNR is increased in part due to thegreater sensitivity achieved by configuring M₂₁ as an amplifier withcurrent gain between the photodiode PD₂₀ and the storage capacitor C₂.The SNR can be further improved by increasing the value of C₂ todecrease the thermal noise associated with M₂₁. The thermal noise forM₂₁ may be approximated as being proportional to 1/C where C is thetotal capacitance from the source node of M₂₁ (node B′) to ground. Thus,by increasing C₂, the thermal noise of M₂₁ decreases.

An additional improvement over the prior art pixel 100 lies in the lower1/f noise (flicker noise) exhibited in transistor M₂₂ of pixel 200, ascompared to that of M₁₂ in the prior art pixel. M₂₂ is a p-channeldevice and so exhibits a 1/f noise several times less than that of M₁₂which is an n-channel device (provided the two devices have comparabledimensions).

The embodiment of the invention as pixel 200 included an output stageB₂₆ based on a p-channel FET M₂₂. Other output stages known to thoseskilled in the art are possible and will not be discussed in thisdisclosure. FIG. 4, however, illustrates another embodiment of theinvention as a pixel 400 which essentially has the same topology aspixel 200 except that the output stage B₄₆ has n-channel FETs M₄₂ andM₄₃ rather than p-channel devices. Using n-channel devices M₄₂ and M₄₃instead of p-channel ones results in a physically smaller pixel 400 ascompared to pixel 200, because the n-channel FETs need not use n-wellsthat would normally be used for implementing the p-channel FETs of pixel200 in a p-substrate.

In pixel 400 the output stage B₄₆ also includes a compensation capacitorC₄₁ coupled between the gate of M₄₂ and the gate of M₄₃ which receivesthe ADDRESS signal. The ratio of C₄₁/C₄ can be adjusted so that thevoltage at node B″ rises by an additional V_(T) or more when the ADDRESSsignal goes to a high level in order to read the OUTPUT node. Forexample, C₄ may be set approximately equal to ¼ of the total capacitanceC₄+C₄₁. Using the compensation capacitor C₄ to raise the voltage at nodeB″ increases the dynamic range of pixel 400, in particular at a lowerend where the voltage at node A″ approaches approximately 1 volt.

The pixel 400 in FIG. 4 otherwise operates in much the same way as pixel200 and in accordance with the timing diagram of FIG. 3 as would berecognized by one skilled in the art, and will therefore not be furtherdiscussed in detail.

The above described embodiments of the invention as pixels 200 and 400may be incorporated into a sensor integrated circuit. The pixels can bearranged in rows and columns to form an array. A column of pixels canhave a common output line such that all of the pixels in the column aremultiplexed to the single output line. In an alternate embodiment, thepixels in a row can be multiplexed to a single output line. In eithercase, the analog output lines from each column or row are fed to ananalog-to-digital (A/D) conversion unit which in turn provides digitalsignals to be further processed according to digital signal processingtechniques. The A/D unit can be part of the sensor IC, or a different ICdepending on the system implementation.

An embodiment of the invention as an imaging system 500 is shown as alogical block diagram in FIG. 5. The imaging system 500 includes anumber of conventional elements, such as an optical system having a lens504 and aperture 508 that is exposed to the incident light reflectedfrom a scene or object 502. The optical system properly channels theincident light towards the image sensor 514 which by virtue of an arrayof pixels 200 or 400 generates analog or digital sensor signals inresponse to an image of the object 502 being formed on the sensor 514.The various control signals used in the operation of the pixels 200 and400, such as RESET, SAMPLE, {overscore (SAMPLE)}, and {overscore(ADDRESS)}, can be generated by a system controller 560. The controller560 may include a microcontroller or a processor with input/output (I/O)interfaces that generates the control signals in response toinstructions stored in a non-volatile programmable memory.Alternatively, a logic circuit that is tailored to generate the controlsignals with proper timing can be used. The system controller also actsin response to user input via the local user interface 558 (as when auser pushes a button or turns a knob of the system 500) or the host/PCinterface 554 to manage the operation of the imaging system 500.

To obtain compressed and/or scaled images, a signal and image processingblock 510 is provided in which hardware and software operates accordingto image processing methodologies to generate captured image data with apredefined resolution in response to receiving the sensor signals.Optional storage devices (not shown) can be used aboard the system 500for storing the captured image data. Such local storage devices mayinclude a removable memory card. A host/Personal Computer (PC)communication interface 554 is normally included for transferring thecaptured image data to an image processing and/or viewing system such asa computer separate from the imaging system 500. The imaging system 500can optionally contain a display means (not shown) for displaying thecaptured image data. For instance, the imaging system 500 may be aportable digital camera having a liquid crystal display or othersuitable low power display for showing the captured image data.

To summarize, the invention has been described in terms of variousembodiments of pixels for use in an image sensor. The pixel embodimentsinclude an amplifier coupled between a photodetector and a storagecapacitor, where a light-generated signal from the photodetector (suchas a voltage across a photodiode) is used to control the amount ofcharge that is placed in the storage capacitor to represent the incidentlight. For instance, the amplifier may be a MOS FET configured as asource-follower which has a voltage gain of less than unity and acurrent gain greater than one.

The embodiments of the invention described above are, of course, subjectto other variations in structure and implementation. For instance, thepixels 200 and 400 feature transistors whose dimensions may be selectedby one skilled in the art in order to achieve proper circuit operationas described above while minimizing power consumption. Also, the valueof the storage capacitor may also be selected by one skilled in the artso as to provide the desired trade off between sensitivity and noiseimmunity, with lower capacitance yielding higher sensitivity but lowernoise immunity. The integration time can also be varied so as to yieldthe desired trade off between pixel resolution and image frame rate.Therefore, the scope of the invention should be determined not by theembodiments illustrated but by the appended claims and their legalequivalents.

What is claimed is:
 1. A method comprising: allowing light to beincident upon a photodetector circuit to thereby generate an inputsignal representing the light, the input signal being applied to a gateof a first field effect transistor (FET); applying a first controlsignal to a drain of the first FET and thereby generating an outputcurrent at a source of the first FET; and accumulating charge in acapacitor coupled to the source of the first FET.
 2. The method of claim1 wherein the first control signal is a digital pulse.
 3. The method ofclaim 2 further comprising: capturing a signal value related to avoltage across the capacitor while the digital pulse is asserted.
 4. Themethod of claim 1 further comprising: asserting a second control signal,that is the complement of the first control signal, to reset thephotodetector circuit; and capturing a reset value related to a voltageacross the capacitor immediately after deasserting the second controlsignal.
 5. The method of claim 4 wherein the second control signal isapplied to a drain of a reset FET coupled to a photodiode in thephotodetector circuit.
 6. The method of claim 3 further comprising:asserting a select signal at a gate of a second FET having a draincoupled to the capacitor through a source follower, to provide a currentrepresentative of the voltage across the capacitor.
 7. The method ofclaim 6 further comprising: raising the voltage at a gate of the sourcefollower in response to the select signal being asserted.
 8. The methodof claim 7 wherein the voltage at the gate of the source follower israised by at least one threshold voltage (V_(t)) in response to theselect signal being asserted.
 9. The method of claim 3 furthercomprising: converting the captured signal value into digital formatrepresenting part of a digital image; and transferring the digitalcaptured signal value to a personal computer.